1. Field of the Invention
The present invention relates to a relay apparatus that is connected to a wire transmission line and a communication apparatus so as to relay communication signals, in a system that performs wire communications, including power line communications using, for example, a home-use power line as a transmission line.
2. Description of Related Art
When using a terminal such as a computer for wire data communications at homes, offices, factories and the like, it is usually necessary to install wires such as cables and terminals to be used as transmission lines at such locations where they are needed. This requires a large scale of installation work before the communication system goes into operation.
However, at homes, offices and factories, commercial power supply, for example, 100V AC-50/60 Hz in Japan, 120V AC-60 Hz in the United States of America is generally used. Therefore, power lines (electric lamp lines) for supplying such power are already installed everywhere within homes, offices and factories. Utilizing these power lines for data communications thus eliminates the need to newly install exclusive wires for communications. More specifically, by simply plugging a communication apparatus into a power outlet, a communication line can be secured.
PLC (Power Line Communication) technology using such power lines for communications is disclosed, for example, in Japanese Laid Open Publication 2000-165304. Currently, various domestic and foreign manufactures are pursuing research and development projects focused on predetermined frequency bands (e.g., 2 MHz-30 MHz in Japan, 1.705-30 MHz in the United States of America). More specifically, cases are envisioned wherein a method such as OFDM (Orthogonal Frequency Division Multiplexing) is used to generate a multicarrier signal using a plurality of subcarriers in order to transmit the multicarrier signal via a power line.
However, indoor power line wiring is very complicated and each building has significantly different conditions. Moreover, since wide-ranging types of electric appliances are connected to such power lines, various noises can occur and induce impedance fluctuations. Therefore, compared to when performing communications via an exclusive wire transmission line, when performing communications via a power line, there is a higher possibility that communication speed slows down considerably or communication quality deteriorates due to decrease in a S/N ratio (signal-to-noise ratio).
In other words, while power line communications are very convenient in that their transmission lines can be easily secured, there are cases in which sufficient communication speed or desirable communication quality cannot be achieved in an environment where various users perform power line communications.
For instance, when an exclusive transmission line is available for supplementary use to substitute a power line, it becomes possible to meet the needs of users who cannot obtain desirable communication speed and quality in power line communications. However, when newly preparing an exclusive wire transmission line, users face situations where wire installation work becomes necessary or where newly installed wires are left exposed in the room. This ruins the benefit of convenience afforded by power line communications.
Mostly, wiring of, such as, for example, a coaxial line that connects a TV antenna and a TV receiver and a telephone line that connects a telephone is normally installed in a standard home. Since exclusive outlets are installed for such wiring, connecting and disconnecting apparatuses is very easy. Therefore, for those who cannot enjoy desirable communication speed and quality in power line communications, existing wiring other than power lines, i.e., coaxial lines for TV antennas and telephone lines, may be utilized as transmission lines.
However, when connecting different electric appliances to the same wiring, an impedance mismatching inevitably occurs and causes problems, since such connection configuration is not standardized. For instance, when connecting a PLC modem to a coaxial line that connects a TV antenna and a TV receiver, the impedance of the modem causes an impedance mismatching on a line between the TV antenna and the TV receiver. As a result, reflections generated in reception signals and other factors deteriorate a S/N ratio for reception signals to be input into the TV receiver. At the same time, due to the influence of the impedance of the TV receiver or the like, an impedance mismatching occurs on the lines between a plurality of modems connected to the coaxial line. As a result, reflections generated in reception signals and other factors deteriorate a S/N ratio, resulting in low communication quality. In addition, signals different from those transmitted by respective electric appliances are received merely as noise. Therefore, when a plurality of different electric appliances shares the same transmission line, the S/N ratio of communication signals deteriorates due to the influence of signals transmitted from other electric appliances or modems. The same results obtain when a telephone line is used as the common transmission line.
As described above, when performing power line communications by connecting a PLC apparatus to existing wire transmission lines other than a power line, such as an antenna line and a telephone line, to utilize these wire transmission lines, there are cases where an impedance mismatching occurs, thereby causing problems, in communications to be performed via these wire transmission lines as well as communications through power line communications, such as lower communication quality. For instance, while a TV receiver connected to an antenna line is receiving a broadcast, TV reception quality may deteriorate due to signal reflections and the like. TV reception signals can also become noise at a PLC apparatus, resulting in lower communication quality.